1. Field of the Invention
The present invention relates to an image forming system including an image forming apparatus having a plurality of fixing units, and a method of controlling the image forming apparatus.
2. Description of the Related Art
Recent color image forming apparatuses that use an electrophotographic technology and form an image forming system apply four color toners of yellow, magenta, cyan, and black to an electrostatic latent image which is obtained by irradiating an image bearing member with a laser beam modulated based on image data, thereby forming toner images. The toner images are transferred to various kinds of printing media (e.g., a PPC paper, OHP sheet, and thick paper; to be referred to as paper hereinafter) and then fused by, e.g., a heat roller. As a fixing unit to fix a toner image on paper, a roll fixing unit or belt fixing unit is employed.
To enhance the gloss of an image after fixing and obtain a fine-looking color image, an image forming apparatus adopts a method of designing an optimum toner viscoelastic characteristic, thereby improving the smoothness of the image after fixing. A full-color electrophotographic printer is used to output image data from a digital camera and is required to form a high-quality high-gloss image.
To meet this requirement, the image forming apparatus uses specialty paper which allows output in a high-gloss mode to obtain a highly glossy image as compared to paper (plain paper) used for normal output. The specialty paper used for output in the high-gloss mode has, on its surface, a transparent resin layer made of a thermoplastic resin. A color toner containing a thermoplastic resin is transferred to the paper surface with the transparent resin layer and heated and fused, thereby forming a high-gloss color image.
To obtain a high glossiness, the image forming apparatus must supply a large amount of heat to paper and toner and fuse the toner. To do this, the fixing speed of paper passing through the fixing unit is decreased to prolong the fixing time of the paper. However, when the fixing speed of paper decreases to prolong the fixing time, the productivity lowers. It is also necessary to decrease the fixing speed and prolong the fixing time in forming an image on specialty paper such as an OHP sheet or thick paper.
An image forming apparatus having a plurality of fixing units has been proposed to avoid the decrease in productivity. For example, Japanese Patent Laid-Open No. 2005-099759 describes an arrangement which has two fixing units with different fixing speeds and conveys paper even to the downstream low-speed belt fixing unit in the high-gloss mode. Japanese Patent Laid-Open No. 10-123863 describes an arrangement which juxtaposes two fixing units, one of which can change the speed for glossiness control.
For an image forming apparatus using one fixing unit, a technique of changing the job sequence has been proposed to avoid time loss and improve productivity. According to an arrangement described in, e.g., Japanese Patent Laid-Open No. 2005-010292, in a process using a plurality of image forming speeds in, e.g., a high-gloss mode, the job sequence is changed so that the images of high-speed jobs are formed at once, and those of low-speed jobs are then formed at once. Japanese Patent Laid-Open No. 2001-274999 describes an arrangement which changes the sequence of pages of image data containing both color and monochrome images in one job, forms the images of color pages at once, and makes them standby in an intermediate tray. Then, the apparatus forms the images of monochrome pages at once and discharges the paper sheets with the formed images in the order of pages.
However, in the above-described image forming apparatus having a plurality of fixing units, the productivity may greatly decrease in image formation of a job that designates an image forming mode using both plain paper and specialty paper such as an OHP sheet, thick paper, or paper having a transparent resin layer on the surface to form a high-gloss image.
More specifically, when output products (products obtained by outputting images onto specialty paper or plain paper) should be stacked on a single discharge unit, the discharged paper stacking sequence of the output products is taken into consideration. In image formation using specialty paper, the start of subsequent image formation must be delayed in consideration of the fixing time or fixing speed.
The reason for this will be described in detail with reference to FIG. 1. For example, assume that in a job for sequentially outputting six images shown in FIG. 1, images 1, 2, 4, and 6 are output to plain paper while images 3 and 5 are output to specialty paper. The two fixing units have different fixing speeds (or fixing times) to cope with plain paper and specialty paper. The fixing speed of the fixing unit for specialty paper is lower than that of the fixing unit for plain paper. Even when the two fixing units are used, fixing on specialty paper takes a longer time than fixing on plain paper. Hence, to output the six images in the order of images 1, 2, 3, 4, 5, and 6, a wait time is necessary before image 4, as shown in FIG. 1, to output image 4 next to image 3. This also applies to image 5. A wait time is necessary before image 6 to output image 6 next to image 5.
For this reason, in a job using both specialty paper and plain paper, if the number of specialty paper sheets increases, the wait time becomes long. Consequently, the productivity of the image forming apparatus decreases.